Recently, significant attention has been focused on fuel cells as clean energy sources capable of highly efficient energy conversion in an environmentally friendly manner. Solid oxide fuel cells (SOFC) are one type of fuel cell that work at very high temperatures, typically between 700° C. and 1000° C. Solid oxide fuel cells can have multiple geometries, but are typically configured in a planar geometry. In a conventional planar configuration, an electrolyte is sandwiched between a single anode electrode and a single cathode electrode. The sandwiched electrolyte is used as a partition between a fuel gas, such as hydrogen, which is supplied to a partition on the anode electrode side, and an air or oxygen gas, which is supplied to the partition on the cathode electrode side.
In a typical solid oxide fuel cell system, approximately one half of the kinetic energy of reactants, such as fuel and oxygen, is converted into electricity and the other half is converted to thermal energy, which causes a significant temperature increase within the SOFC system. In order to trigger fast electrochemical reactions, the reactants often must be heated to a high temperature. For example, in a system using a thin yttria-partially stabilized zirconia (3YSZ) electrolyte, the reactants have to be heated to approximately 725° C. to obtain an effective reaction. With such an initial temperature of reactants, the peak temperature within the fuel cell for a stoichiometric hydrogen-air system can rise to more than 1000° C.
The electrical and mechanical performance of fuel cells depends heavily on the operating temperature of the system. At high temperatures (such as about 1000° C. or more), serious issues may arise in the way of thermal mechanical stress and the melting of sealing materials within the solid oxide fuel cell system components. Furthermore, external heating is often needed to heat the reactants to their optimal reaction temperature, which results in low overall system efficiency.
Various thermal management strategies have been developed. For example, U.S. 2004/0170879A1 discloses a shape memory alloy structure that is connected to a fuel cell for thermal management. U.S. 2005/0014046A1 discloses an internal bipolar heat exchanger that is used to remove the heat from an anode side of an individual cell to heat the cathode flow of another cell. In U.S. 2004/0028972A1, a fluid heat exchanger is disclosed for transferring heat between fuel cell units and a heat exchanger fluid flow, which flows in a direction perpendicular to the electrolyte surface. Further, in U.S. 2003/017695A1, a reformer reactor is disclosed that is connected to a fuel cell for helping the thermal management at the system level. In WO2003065488A1, an internal reformer is disclosed for use in thermal management of a fuel cell.
Accordingly, there is a need in the art for thermal management systems and methods that are able to both reduce the thermal mechanical stress that results from the thermal energy generated in the reaction and preheat the reactants that enter the reaction chamber increase the overall system efficiency of the solid oxide fuel cell